1. Field of the Invention
The present invention concerns a method of releasing from a mold transparent polymer material articles such as ophthalmic lenses (in particular spectacle lenses) made by pouring and polymerizing a polymerizable liquid substance into the cavity of a two-part mold.
2. Description of the Prior Art
Transparent polymer material (organic glass) ophthalmic lenses are conventionally made by pouring and polymerizing a polymerizable liquid substance into the cavity formed between two parts of a mineral glass mold, the assembly being held together by fixing means such as clips. The assembly is commonly called a "sandwich".
After polymerization the fixing means are removed and the mold is dismantled to recover the ophthalmic lens.
The polymerizable liquid substances used vary widely, depending on the required refractive index of the ophthalmic lens.
Many of these substances, for example substances comprising a monomer such as diethylene glycol bis(allylcarbonate) (CR39.sup.200), shrink significantly on polymerization, by an amount in the order of 7% or more, generally 10% or more, creating tensile stresses in the ophthalmic lens within the sandwich.
Some ophthalmic lenses, in particular lenses with a positive optical power, can have random defects that are visible when they are subsequently colored, for example by dipping them into an aqueous dispersion of pigment. Because of their shape these defects are commonly called "ferns".
The defects are probably due to sudden relieving of stresses during mold release, especially in the case of substances which shrink greatly on polymerization and positive optical power ophthalmic lenses.
U.S. Pat. No. 3,171,869 describes a method of releasing from the mold ophthalmic lenses obtained by polymerizing a liquid substance, in particular one which shrinks greatly on polymerization, which consists in heating the sandwich to a temperature in the range from 125.degree. C. to 235.degree. C. just before polymerization is completed to soften the material of the lens and simultaneously relieve the polymerization stress. During this heating the polymerized material separates from the surface of the mold.
The mold release method of U.S. Pat. No. 3,171,869 reduces the risk of breaking molded lenses when releasing them from the mold. However, the molded lens inevitably separates from the surface of the mold on heating or shortly after cooling and this induces stresses that can lead to the formation of ferns.
Document WO 97/39880 describes a method of making lenses including a step of transferring heat by conduction from the surface of the mold in order to increase the degree of polymerization of the surface of the lens to obtain a more homogeneous distribution of color over the surface of the lens during subsequent coloring.
Because heating is effected by conduction of heat from a surface of the sandwich, the sandwich is not heated uniformly and this can induce stresses in the polymerized lens.
It would therefore seem desirable to develop a method of releasing from the mold transparent polymer material articles such as lenses obtained by polymerizing a polymerizable liquid substance in the cavity of a two-part mold that significantly reduces the risk of ferns and other defects occurring during subsequent coloring of the lenses.